Only in unique environmental situations do species of organisms live entirely alone. Communities almost always have mixed populations. Due to this variety, many types of interactions, some beneficial, others not, are possible due to the members of an ecosystem's (fresh water or salt-water) community. Symbiosis, the intimate living together of two or more kinds of plants and/or organisms in a mutually beneficial environment has played a key role in the evolution of many species.
In natural environments, many types of symbiosis are easily discernible, for example, the rhinoceros and parasite feeding birds, the shark and the pilot fish. Aphids suck phloem, removing certain amino acids, sugars and other nutrients from it while excreting most of it as honeydew or sugar-lerp, where it is harvested by the Aborigines in Australia. Some species of aphids have been domesticated by several species of ants. These aphids do not excrete their honeydew continuously, but only in response to caressing movements of the ant's antennae and forelimbs. The aphids involved in this symbiotic relationship with the ants have lost all of their own natural defenses, including their hard outer skeletons. The aphids rely entirely on their hosts for protection. In other cases, the relationship is not readily apparent, such as the leaf-cutter ants and bacteria that produce antibiotics that increase the survivability of the ant host.
Commercial fish harvesters use a variety of amendments to enhance fish growth. Some include vitamins and other nutrients to maintain good health and to help fish resist diseases. Other methods include genetic alteration and manipulation.